I use a white gel pen to "hide" it. It's still glaringly obvious to me but... well, that's the only trick I know. I put several layers on, not just one, because the first layer comes out a little grey. Sometimes I'll add more dots around it, and make the white paper like a white-polka-dot paper.

Thanks for this great tip! I had a tiny ink spot on a card, so tried this and it worked beautifully! It only took a couple of layers of the white gel ink before that spot disappeared. Awesome tip!